Technical Field
The present invention relates generally to the detection of a target material contained within a sample material. More particularly, the present invention relates the detection of explosives contained in a sample material. Specifically, the present invention utilizes Nuclear Quadrupole Resonance to locate the target material so it may be evaluated and determined whether it is an explosive.
Background Information
Since the earliest days of explosive and contraband detection, people have been trying to gain advantages over their opponents. These advantages have become more desirable ever since the advent of extremely deadly explosives, and highly illegal contraband. One desirable advantage is the ability to remotely detect an explosive within a set of sample material. This sample material could be in the ground, so as to detect an explosive mine or the sample material may be separated from the ground, so as to detect an explosive bomb or contraband contained in a box.
Prior to describing the subject invention and by way of further background, in the early 1900s, not long after Einstein published his equations on thermal equilibrium, individuals realized that there were likely to be resonances at very low frequencies for atoms and molecules and that these resonances would occur because if one emits a photon of exactly the correct frequency, the material will absorb this photon, store it for some amount of time and then get rid of the absorbed energy. It is has been found that in nature the molecules which absorb such energy always fall to a lower energy state.
One of the ways for the material to emit energy is through spontaneous emission where a photon of exactly the same energy that is impinging on the material is thrown off in a random direction at random times.
The second way of getting rid of the energy absorbed by the material is through process of stimulated emission in which a photon arrives at exactly the appropriate energy, gets near the molecule, stimulates the molecule and when the molecule drops to the lower energy state it emits a photon that is exactly in phase with the original photon.
The energy that is thrown off either in spontaneous emission or stimulated emission results in a narrow spectral line. In fact, the line is generally considered to be a single line that exists at a given wavelength or frequency.
Nuclear quadrupole resonance (NQR) is a branch of radio-frequency spectroscopy (which includes the study of spectral lines). NQR has been utilized in the past to detect the presence of specific molecules, including explosives. Explosives generally involve the use of nitrogen or nitrogen bonded with other elements. When NQR was utilized in the past, it was used to detect the presence of molecules due to the molecular elements that are bonded together such that the molecules absorb energy at, for instance, as many as eight different energy levels or spectral lines. It turns out that at least three of the energy levels tend to be prominent. Although in some materials, there are upwards of all eight energy levels for one bond. If one has many bonds, there may be many dozens of spectral lines. In order to detect the presence of a molecule one usually is looking to pump energy right at the top of one of the spectral lines and look for energy coming back at the same frequency.
A further background explanation of NQR was originally published in U.S. Pat. No. 6,194,898 ('898 patent). The '898 patent explains that NQR exploits the inherent electrical properties of atomic nuclei. Nuclei with non-spherical electric charge distributions possess electric quadrupole moments. Quadrupole resonance arises from the interaction of the nuclear quadrupole moment of the nucleus with the local applied electrical field gradients produced by the surrounding atomic environment. NQR does not require an external static magnetic field as NMR does.
Any chemical elements nucleus having a spin quantum number greater than one half can exhibit quadrupolar resonance. Many substances (approximately 10,000) have been identified that exhibit quadrupolar resonance, among such nuclei being: 7Li, 9Be, 14N, 17O, 23Na, 27Al, 35Cl, 37Cl, 39K, 55Mn, 75As, 79Br, 81Br, 127I, 197Au, and 209Bi. It so happens that some of these quadrupolar nuclei are present in explosive and narcotic (i.e., contraband) materials, among them being nitrogen (14N), chlorine (35Cl, 37Cl), oxygen (17O), sodium (23Na), and potassium (39K). The most studied quadruple nucleus for explosives and contraband detection is nitrogen.
In solid materials, electrons and atomic nuclei produce electric field gradients. These gradients modify the energy levels of any quadrupolar nuclei and hence their characteristic transition frequencies. Measurements of these frequencies or relaxation time constants, or both, can indicate not only which nuclei are present but also their chemical environment.
When an atomic quadrupolar nucleus is within an electric field gradient, variations in the local field associated with the field gradient affect different parts of the nucleus in different ways. The combined forces of these fields cause the quadrupole to experience a torque, which causes it to precess about the electric field gradient. Precessional motion generates an oscillating nuclear magnetic moment. An externally applied radio frequency (RF) magnetic field in phase with the quadrupole's precessional frequency can tip the orientation of the nucleus momentarily. The energy levels are briefly not in equilibrium, then the energy levels immediately begin to return to equilibrium. As the nuclei return to equilibrium, they produce an RF signal, known as the free induction decay (FID) or return frequency. A pick-up coil detects the signal, which is subsequently amplified by a sensitive receiver to measure its characteristics.
One distinguishing of an NQR response is the NQR relaxation times. Relaxation times are a measure of the nuclei's rate of return to the equilibrium state following disturbance by an RF irradiation pulse. Relaxation times are compound-, temperature-, and pressure-specific. Relaxation times also determine the repetition rate and timing of RF pulses required for exciting and detecting a specific NQR signal. Relaxation times from pulsed systems can be as long as eight seconds for some materials like TNT.
The '898 patent discloses a method for detecting a target substance within a class of explosives and narcotics containing quadrupolar nuclei in a specimen, said method employing the phenomenon of nuclear quadrupole resonance (NQR) in a pulsed detection system and comprising the steps of: forming a scanner having an RF Coil for a probe; entering known characteristics of NQR signals of target substances in memory in a signal processor in the detection system; providing programmed timing pulses to the detection system; inserting the specimen within the volume enclosed by the RF coil; then automatically adaptively tuning the RF coil to maximize power transfer efficiency for RF signals transmitted within the RF coil cavity; providing excitation RF pulses of a predetermined frequency to the RF coil; transmitting the RF pulses into the cavity formed by the RF coil and creating a flux field with the RF coil to which the specimen is subjected; detecting by the RF coil the NQR signals emitted by target substances within the specimen; processing the NQR signals and comparing them to known signal characteristics to determine whether the detected NQR signals indicate the presence of a target substance; and indicating whether the target substance is present in the specimen.
Problems and issues may still arise with the '898 patent, namely, with respect to the detection of explosives or other contraband. One problem is that certain non-explosive materials also contain nitrogen bonds having energy levels substantially equal or in a similar frequency range as an explosive or contraband material. An additional problem results from the RF pulses. The RF pulses require a large amount of power. The amount of power needed to operate the system of the '898 patent is 1 to 2 KW RF power amplifier for examining airline baggage for explosives. This system would be unsafe for direct human use.
Further, an issue arises where the RF pulses would require a relaxation time (the time between the broadcasted pulse signals and a receiver being able to locate a return frequency signal) in the range of about 3 to 8 seconds for detecting trinitrotoluene (TNT). This relaxation time is too long for any real world application requiring scanning of moving people. This is because the device would have to stop irradiating signals and “listen” for the NQR emission or return frequency until sample material would stop resonating prior to device sending out another pulse. Or, this problem would also arise when the pulsed system would be transmitting signals while simultaneously trying to measure or listen for the return frequency.
Another issue with pulsed NQR detection systems is there is a large dynamic range in decibels (db, the ratio between two values of a physical quantity, power and intensity [amplitude]). The dynamic range can be as large as 150 db to measure a return frequency for a target material within a sample material. For example, to locate a nitrate in a sample material, an electromagnetic pulsed signal is transmitted/broadcasted at 0 db (1 to 1 ratio) into the sample material. The expected NQR emitted signal from any nitrate would be in an expected range from about −125 db to about −150 db. This is an extremely low level of return signal strength, making it difficult to positively identify the nitrate.
A United States Patent Application Publication 2009/0039884 ('884 application), published in the name of Schiano, discloses a method of using a method of characterizing a nuclear quadrupole resonance for an analyte within a sample volume using continuous wave (CW) spectroscopy, the method being performed by a spectrometer, the method comprising: applying an excitation magnetic field having a search frequency to the sample volume; adjusting the search frequency using a blind search algorithm to detect a resonance absorption signal; and adjusting the search frequency using an extremum seeking algorithm so as to determine an extremum in the resonance absorption signal.
The '884 application is remarkable in that problems may arise with respect to searching for target material. The '884 application may still have relaxation times in the range of 3-8 seconds for target materials, such as TNT, because the '884 application does not disclose any way of suppressing the transmitted excitation signal from overdriving or jamming the receiver detection circuit. Further, it may be unsafe for human use depending on the total power irradiating the sample material.
The present invention addresses these and other issues.